This commit explains how to calls a method implemented in a language
from a different language.
This explains how to declare the RPCs, how to call them and how to
implement them. This is based on examples of code at main at the time
of writting. I used permalink to ensure that the links remains
relevant even if the specific examples change later.
The last section is about the special case of calling TypeScript from
Python, which does not use RPC but is still relevant in a bridge
document.
This commit also add a paragraph explaining what protobuf is in the
protobuf documentation, so that new contributors who don't know what
protobuf is can understand why we use it.
